An embodiment is more particularly directed to a Fourier spectrometer comprising:                a coherent light source;        an interferometer adapted to separate, by amplitude or wavefront separation, the coherent light source into two parts in order to generate interferences between the two parts;        detection means adapted to detect the interferences.        
Such Fourier spectrometer is described e.g. in “Femtosecond laser Fourier Transform absorption spectroscopy”, Mandon et al. OPTICS LETTERS, Jun. 15, 2007 Vol. 32, No. 12 p. 1677-1679, which is incorporated by reference. In this document the coherent light source is a Cr4+:YAG femtosecond mode-locked laser.
However, the use of the Cr4+:YAG femtosecond mode-locked laser as light source in the aforementioned document has several drawbacks. The repetition rate of the mode-locked laser is not constant and the phase shift between the pulses of the mode-locked laser is not stabilized. As a consequence, the spectrum of the mode-locked does not exhibit a comb-like structure made of sharp and equidistant frequency markers. Because this comb structure is missing, it is necessary to know the spectrum of some reference species of the sample in order to calibrate the spectrometer. Furthermore, interferences can only be measured between a frequency of the mode-locked laser and the same frequency shifted by Doppler effect. Therefore, the noise level of the spectrometer is limited by the fringe scanning rate, typically in the audio domain, produced by the interferometer and by the amplitude noise of the mode-locked laser. In addition, such spectrometer can not provide simultaneous access to the absorption and dispersion experienced by the sample.
An embodiment provides an improved Fourier spectrometer which is capable of overcoming the aforementioned limitations of the known art.
An embodiment is based on the idea that frequency comb generators, which are mainly used in the context of metrology, can provide satisfactory stabilized pulses, overcoming the above cited drawbacks.
Accordingly a first embodiment relates to a Fourier transform spectrometer comprising:
                a coherent light source;        an interferometer adapted to separate the coherent light source into two or more parts in order to generate through frequency- or phase-induced effects, interferences between the two or more parts;        detection means adapted to detect the interferences,wherein the coherent light source comprises a frequency comb generator having a frequency repetition rate, and in a particular embodiment, the detection means may be adapted to detect the beating of pairs of frequencies of the frequency comb separated by the frequency repetition rate or a multiple of the frequency repetition rate.        
In the prior art, the detection was made from the beating of a certain frequency with the replica of this frequency, which is frequency-shifted by Doppler-effect or phase-shifted effect produced by the moving arm of the interferometer. With an embodiment, the stability of repetition rate of the frequency comb is advantageously used to make the detection from pairs of frequencies separated by the frequency repetition rate or a multiple of the frequency repetition rate, i.e. N times the frequency repetition rate with N≧1.
With this new type of detection, the noise level of the interferogram is decreased by at least 1000.
The interferometer may be an interferometer based on amplitude or wavefront separation of the incoming source into two beams. For instance, it may be a Michelson interferometer, which comprises in its simplest optical configuration a fixed mirror, a moving mirror and separating means adapted to separate the coherent light source into two parts, the first part being adapted to reflect on the fixed mirror, and the second part being adapted to reflect on the moving mirror. All traditional interferogram recording procedures (one or more or no moving mirror, one or more or no fixed mirror, linear or not variations of the path difference producing the interferogram) are usable.